Ion sensitive electrode and cells for use therewith

ABSTRACT

An ion sensitive electrode comprises a small disc of ion selective polymeror contact with a measuring liquid, the ion selective polymer disc being in contact with a wire of low drift potential relative to the ion selective polymer disc. The invention also includes an electro mechanical measuring arrangement in which the ion sensitive electrode may be used and also a reference electrode suitable for use therewith.

This is a division of application Ser. No. 652,349, filed Jan. 26, 1976and now U.S. Pat. No. 4,135,999.

BACKGROUND OF THE INVENTION

The invention relates to an ion selective electrode in which an ionselective polymer layer is in contact on the one hand with the measuringliquid and on the other hand with a wire with low drift potentialrelative to the polymer layer.

Recently, ion selective electrodes have gained considerable importancefor electrochemical measurement of various types, particularly inphysiology (cf. K. Cammann, "Das Arbeiten mit ionenselektivenElektroden", Springer-Verlag, Berlin, Heidelberg, New York, 1973). Solidmembrane electrodes and liquid membrane electrodes are distinguishedfrom one another. The wire-clad electrodes may be regarded as a furtherdevelopment of the latter (cf. K. Cammann, page 106). They consist of athin platinum wire which is coated with a layer of polyvinylchloride(PVC). The PVC layer is saturated with the active phase, i.e. the ionselective substance. These electrodes are manufactured with an innerelectrolyte in a simple manner in camparison with the normal liquidmembrane electrodes, and are also layer or position independent. Theselectivity ratio is more favourable than in corresponding liquidmembrane electrode types, the pick-up time is very short and the lifespan is considerably higher than in normal liquid membrane electrodes.

SUMMARY OF THE INVENTION

It is an object of the invention to further improve such electrodes withrespect to their reproducibility, the possibility of miniaturizing them,and their versatility of application.

It is a further object of the invention to provide a reference electrodesuitable for use with the ion selective electrode of the invention inwhich the reference electrode is re-shaped so that a stable diffusionpotential can be achieved despite a very small contact area.

It is yet a further object of the invention to provide a single rodmeasuring cell with an ion sensitive electrode and a referenceelectrode, suitably electrodes in accordance with the invention, whichcan be used with simple means both for flow measurement and stationarymeasurement.

According to a first aspect of the invention, there is provided an ionselective electrode comprising a small disc of ion selective polymer forcontact with a measuring liquid, a wire with a low drift potentialrelative to said ion selective polymer disc and a face of said wire incontact with said ion selective polymer disc.

According to a second aspect of the invention, there is provided areference electrode for use with an ion selective electrode comprising acontact making electrolyte and a small PTFE plate defining an opening ascontact area and so small that said contact making electrolyte can flowout of said opening, at normal pressure and room temperature at a rateless than 5×10⁻⁶ μl/sec.

According to a third aspect of the invention, there is provided a singlerod measuring cell comprising an electrode body, a reference electrodeat one surface of said electrode body, a measuring electrode at the samesurface of said electrode body as said reference electrode but spaced asmall distance therefrom, an elongated tub-shaped measuring chamberdelimited by said one surface of said electrode body and inflow andoutflow channels leading into the narrow ends of said measuring chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail, by way ofexample, with reference to the drawings, in which:

FIG. 1 shows a perspective view of a single-rod measuring cell with thenew electrodes;

FIG. 2 shows a longitudinal section of the same;

FIG. 3 shows a phantom diagram of the removable casing, and

FIG. 4 shows a bottom view of a modified embodiment of the measuringcell with the casing removed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Basically, the invention proposes that the polymer layer of an ionselective electrode is constructed as a small disc abutting the surfaceof the wire.

The wire surface to be coated with the selective substance is thereforediminished at the free surface of the wire--preferably a platinum wire.This miniaturization facilitates bunching of many electrodes in thesmallest space.

The small disc consisting of the polymer may overhang the outer surfaceof the platinum wire in order to increase the contact area with themeasuring liquid.

Thanks to the small area necessary several electrodes coated withvarious selective substances may be housed together in a very smallspace next to one another in a common measuring chamber, so that severalion activities may be determined in a single operation.

The new electrode is suited, because of its small dimensions,particularly for construction as a catheter electrode or for attachmentto areas of a very few millimeters, e.g. in a living body.

Furthermore, the new electrode is suited, because of its smalldimensions, particularly well to construction of a so-called single-rodmeasuring cell i.e. construction of an electrochemical cell in which themeasuring electrode and the reference electrode are housed close to oneanother. These single-rod measuring cells are particularly suitable forflow measurement. Of course, here there is the difficulty that the knownreference electrodes need a considerable amount of space in comparisonto the new measuring electrode so that the miniaturization being strivenfor is in this respect hindered.

In fact, reference electrodes having a capillary contact area with themeasuring liquid are known; however, in practice these have not givensatisfactory results because they show unstable diffusion potentials.Therefore, in the professional world only those reference electrodes arerecommended in which the contact making electrolyte providing thecurrent path to the connection wire may pass into the measuring liquidthrough a non-lubricated ground connection (see the said book byCammann, page 47). Therefore, it must be taken into account that theoutflow rate of the contact making electrolyte is relatively large andtherefore makes the measuring solution dirty and even poisons it.Furthermore, the contact making electrolyte must be suppliedcontinuously.

Accordingly, therefore, in a specific embodiment of the invention, it isdesired to re-shape the known reference electrodes so that a stablediffusion potential is achieved despite a very small contact area.

It was found that this may be achieved if the contact area consists of amicrobore in a small plate made of polytetrafluoroethylene (PTFE).

The microbore may, for example, be formed so that a micronotch isarranged on the side wall of a stopper made of PTFE and that then thisstopper is pressed into a corresponding opening in an electrode bodyconsisting of PTFE. Another solution consists in welding a membrane madeof PTFE on to the electrode body, the membrane being provided with afine hole, for example by means of a laser beam. The microleak thusformed is so small that the outflow rate of the reference solution isless than 2.6×10⁻⁶ μl/sec in normal pressure conditions. Thisconstruction of the contact area has proved to be rarely faulty asagainst previous experiences in the professional world, so that thereference potential could be successfully produced even with longer use.

The contact area, formed by the outlet opening, of the referenceelectrode thus manufactured may lie at a distance of less than 1 mm fromthe measuring electrode so that an extremely compact arrangement of asingle-rod measuring cell may be created.

Use of the described reference electrode is, however, not limited to usetogether with the new measuring electrode, but may take place toadvantage with conventional single and multi-rod measuring cells.

Single-rod measuring cells are in fact preferably used for flowmeasurement but are also advantageous for stationary measurements. Thisis valid all the more for single-rod measuring cells withmulti-electrodes. In flow measurement, particularly in the physiologicaland pathophysiological region, it is frequently necessary to work withvery low amounts of electrolyte. Thus it is desired, in preferred formsof the invention, to develop a single-rod measuring cell with themeasuring and reference electrodes described above which may be put intouse with simple means both for flow measurement and for stationarymeasurement and in which it is ensured that a measuring solutionavailable only in small amounts comes into intimate contact with thecontact zones of the electrodes.

For this purpose, according to an embodiment of the invention anelongated tub-shaped (bathtub-shaped) flow chamber is constructed in theregion of the contact areas of the electrode body surrounded by thecasing at the inner wall of the floor of a removable casing and the flowchamber is accessible via radial channels from the outer wall of thecasing. If, therefore, the casing is removed, the single-rod measuringcell may be plunged into a beaker or the like with the measuringsolution. If, on the other hand, the casing is pushed over the electrodebody and is fixed for example by means of a retaining nut or a bayonetcatch, then the edges of the elongated tub-shaped flow chamber sealagainst the outer surfaces of the electrode body and the measuringliquid may be supplied and removed through the channels, wherein saidfluid comes into intimate contact with the entire measurement areathanks to the flattening of the flow chamber.

Referring now to the drawings, the main members of the shown measuringchain are an electrode head 1 with holes for an electrode plug 2 and asupply pipe 3 for the reference electrolyte, a casing 4, which is fixedto the electrode head 1 by means of a retaining nut 5 with a screwthreador a bayonet catch, and an electrode body 6 (FIG. 2) located in thecasing. In the base member of the casing 4 are located pipe attachments7 and 8, which act to supply and remove the measuring liquid.

The electrode head 1 and the retaining nut 5 comprise for example,polyacryl or metal, the casing 4 is made of polyacryl and the electrodebody 6 is made of PTFE.

As may be seen from FIG. 2, the electrode body 6 has two cylindricallongitudinal bores 9 and 10. The bore 9 acts to accommodate thereference electrode 11 made of Ag/AgC₁, which passes through a stopper12 at the upper end of the electrode body and is connected by means of aconnecting wire 13 to the earth connection 14 of the coaxial plug 2. Thestopper 12 is also penetrated by the end 15 of the delivery pipe 3through which the contact making electrolyte (usually KCl) of thereference electrode may be supplied.

In the embodiment shown in FIG. 2, the bore 9 does not quite extend atits full width to the lower surface 16 of the electrode body 6, but iscontinued in a narrower aperture 17. This aperture 17 is closed by astopper 18 made of PTFE which is pressed in and which is provided at onepoint on its circumference with a small longitudinal notch 39 the depthof which is not more than 0.1 mm, preferably less than 0.5 mm. The notchhas such a flow resistance that the outflow rate of the referencesolution is not more than about 5×10³¹ 6 μl/sec under normal pressureconditions, and is preferably less than 2.6×10⁻⁶ μl/sec.

The longitudinal bore 10 has a low unobstructed width (e.g. 2 mm), as itdoes not have to accommodate any liquid. On its lower end there islocated a stopper 19 made of polyacryl which has been bored through, inthe longitudinal bore of which is inserted a platinum wire 20. Aconnecting wire 22 is soldered on to the inner face 21 of the platinumwire, said wire 22 leading to the inner lead 23 of the connecting plug2.

The stopper 19 does not quite reach to the face 16 of the electrode body6, but is set back from this by about 0.1 to 1 mm. The platinum wire 20is also set back by a further small distance from the lower face of thestopper 19. A solid ion selective substance 24 is located in thedepression thus formed. It consists peferably of a polymer matrix (e.g.PVC) in which the active phase is incorporated. Thus, for example, asolution of PVC in cyclohexanone is mixed in a suitable ratio (15%) withthe active phase which is selective for the desired ions and the mixtureis poured into the depression and dried.

An intimate contact of the selective substance with the wire is assuredby the small distance that the platinum wire is set back and the dangerof a direct contact of the wire with traces of leakage of the measuringliquid is reduced. Instead of platinum, another material having lowpotential drift may be used for the wire 20.

The face 16 of the electrode body 6 forms one limit of a flow chamber 25constructed in the base of the casing 4. It has the shape of anelongated tub which is about 0.6 mm deep, the length and width of whichare chosen so that the contact areas of the measuring electrode and thereference electrode lie completely in the measuring chamber. If, as inthe present example the distance of the two contact areas is about 1 mmand the diameter of the contact area of the measuring electrode is about2 mm, the width of the measuring chamber may also be 2 mm and its length5 mm.

Radial channels 26 and 27 open in the centre of the narrow sides or endsof the measuring chamber 25, which are bored from outside so far intothe casing closely below the inner wall of the casing floor that themeasuring chamber 25 is just cut. Attached to the channels 26 and 27 arethe outer supply and removal pipes 7 and 8. Preferably the measuringliquid is supplied via the channel 27 and removed via the channel 26, sothat first the measuring liquid comes into contact with the measuringelectrode, and then with the reference electrode, so that anyinfluencing of the result of measurement by the reference electrolytemay be avoided.

The cross-section of the measuring chamber 25 is approximately the sameas that of the channels 26 and 27 so that the measuring liquid passesthrough at an even or constant speed yet comes into abundant contactwith the entire contact area as a result of the flat construction of themeasuring chamber.

In order to ensure that the casing is correctly pushed over theelectrode body, so that the edges of the measuring chamber seal bothelectrodes, the floor of the casing 4 is provided with projections 28(FIG. 3) on both sides of the measuring chamber, these projectionsfitting into corresponding recesses 29 (FIG. 4) in the electrode body.

FIG. 4 shows yet another modification of the embodiment of the referenceelectrode: instead of the stopper 18, here a thin membrane 30 made ofPTFE is welded on to the face 16 of the electrode body in order to coverthe opening 17. In the membrane 30 there is a small aperture 31 ofapproximately 0.01 mm φ at the most, formed mechanically orelectronically. This aperture size makes it possible to keep to theabove-stated outflow speed also in this embodiment.

The described measuring chamber construction has proved to beinsensitive to oscillations in the flow speed so that the measuring cellmay be operated pulsatingly even with a simple pipe roller pump.

EXAMPLE

The calcium salt of didecyl phosphoric acid in dioctyl phenylphosphonate was chosen as an active substance. The active phase wasmixed in the above-described manner with PVC and the mixture wasinserted into the a flat bowl depression of the measuring electrode.After drying, the Ag/AgC₁ reference electrode was filled with 3-molarKCl solution and the casing was placed on top. Thus a calcium selectivesingle-rod measuring cell is obtained.

The measuring chamber was connected to a liquid supply so that themeasuring material at first passes the measuring electrode. Acommercially common pipe roller pump was used as the pump.

First of all, the activity potential was determined in pure CaCl₂solutions by measuring with different concentrations. The theoreticalmeasurement characteristic of the electrodes (Nernst-factor) fordi-valent ions is 29.083 mV/activity decade at 20° C. with the describedflow metering chamber, a potential difference of 28.0 mV was measuredbetween 10⁻² and 10⁻³ mol CaCl₂ /l.

The speed of indication is approximately 30 sec/activity decade in pure10⁻² and 10⁻³ mole solutions. In these conditions a measurement accuracyof ±0.4 mV and better could be achieved.

The selectivity was determined according to the principle of changingthe measured ion activity when the activity of interfering ions ismaintained constant. Aqueous solutions of 10⁻² and 10⁻³ mole CaCl₂ /lwere added as a chloride to the concentrations of interfering ions of150 mval Na/l or 150 mval K/l or 150 mval Mg/l. Taking into account thenon-ideal relationship of the solutions used, the following selectivityconstants were ascertained from the measured potential changes:

    K.sub.Ca-Na =9.5×10.sup.-3, K.sub.Ca-K =3×10.sup.-4, K.sub.Ca-Mg =1.8×10.sup.-3.

In contrast, in the literature on the subject (see Cammann, page 98) thefollowing values are found for liquid membrane electrodes with the sameactive phase:

    K.sub.Ca-Na =10.sup.-3, K.sub.Ca-K =10.sup.-3, K.sub.Ca-Mg =14×10.sup.-3.

The measurements of the potential were carried out with the digital pHmeter E 500 by the Metrohm company (Herisau, Switzerland).

The described electrode and also the measuring cell constructedtherewith are particularly suitable as one-time and disposableelectrodes because of their simplicity and relatively cheap manufacture.

It will be understood that the above description of the presentinvention is susceptible to various modification changes andadaptations.

What is claimed is:
 1. A single rod measuring cell comprising anelectrode body having one flat surface, a reference electrode and ameasuring electrode flush with said surface but spaced from each otherby a small distance, and including a casing surrounding said electrodebody defining a tub-shaped measuring chamber, delimited by said flatsurface of said electrode body and by a flat inner wall opposite saidflat surface disposed proximate one end of said casing, said measuringchamber further delimited by two narrow ends, said casing furtherincluding inflow and outflow channels being bored from the outside ofsaid casing inwardly just below said inner wall of said casing, andending at said two narrow ends of said measuring chamber, said narrowends just being cut thereby
 2. A single rod measuring cell according toclaim 1, wherein the length and width of said elongated tub-shapedmeasuring chamber is substantially of the same dimensions as that ofsaid measuring and reference electrodes at said flat surface of saidelectrode body.
 3. A cell as defined in claim 2, wherein the crosssection of said elongated tub-shaped measuring chamber is substantiallyequal to that of said inflow and outflow channels.
 4. A cell as definedin claim 1, wherein said casing further includes electrical contactmeans for said reference electrode and said measuring electrode in anend of said electrode body said electrode body being in communicationwith said elongated tub-shaped measuring chamber.
 5. A cell as definedin claim 1, wherein said casing is removably mounted on said electrodebody.
 6. A cell as defined in claim 5, further including matingprojections and recesses in said electrode body and said casing adaptedto cooperate with each other for accurately locating said measuringchamber with respect to said electrodes.